1. Field of the invention
This invention involves the application of tunable radiation sources to remotely sense objects with opaque surfaces.
More specifically, this invention addresses the systems and methods to extract information about the size and shape of an object by observing variations of the radiation pattern caused by illuminating the object with coherent radiation sources and changing the wavelengths of the source.
2. Description of the Prior Art
Multiple technologies and methods are used to sense and construct multi-dimensional representations of remote objects. Common three-dimensional imaging solutions include triangulation-based laser scanners, scanning modulated waveform laser radars and imaging time-of-flight laser radars. However, each of these technologies has limitations.
Triangulation-based laser scanners direct a laser beam at the surface from one end of the scanner and receive the reflected laser light at the other end of the scanner. By measuring the angle, the distance can be calculated. Although capable of high accuracy, they are of limited use for long range scanners, because the longer the range, the larger the scanner needs to be. Shadowing effects also impact the ability of this technique to construct accurate representations of objects.
Scanning modulated waveform laser radars modulate a laser beam in a known way and record the returning beam. By comparing the outgoing and incoming waveforms, the distance can be computed. This technique is a relatively slow point-by-point measurement technique requiring the beam to scan the object. This technique provides limited resolution.
Imaging time-of-flight laser radars direct a laser beam out towards the object, then measure how long it takes for the light to return. By using the time taken for the response and the speed of light, the distance can be calculated. Although time-of-flight solutions are capable of operating over very long distances, due to the high velocity of light, timing the round-trip time is difficult and the accuracy of the range resolution is relatively low.
It is also known that “speckle” can be used to obtain three-dimensional information about a remote object. “Speckle” is an interference phenomenon that occurs when coherent radiation (e.g., laser light) is reflected from a rough or multiply scattering sample onto a detection plane. Due to scattering of photons from and within the sample, different photons travel different distances to the detection plane. As a result, the light reflected or backscattered from the sample, if temporally coherent, interferes at the detection plane, producing a grainy pattern known as “speckle.” Techniques exist to utilize speckle patterns, called speckle-pattern sampling, to obtain range information from the wavelength of speckle as well as measure the spatial dependence of the speckle pattern to resolve the object laterally. These speckle patterns can be detected with a sensor array at each of a set of equally spaced laser frequencies. The individual frames are stacked to form a three-dimensional data array, and a three-dimensional Fourier Transform (FT) is performed on this data array. The FT process yields the three-dimensional autocorrelation function of the three-dimensional image of the object. The use of “speckle” for three-dimensional imaging is a technique disclosed in U.S. Pat. Ser. No. 5,627,363 which is herein incorporated by reference. Techniques for speckle-pattern sampling are also described in L. Shirley and G. Hallerman, “Technical Report 1025, “Application of Tunable Lasers to Laser Radar and 3D Imaging,” MIT Lincoln Lab Technical Report 1025, 26 Feb. 1996, which is herein incorporated by reference.
Although speckle-pattern sampling is an improvement in the art of three-dimensional imaging, there are still shortcomings to this technique. With speckle-pattern sampling, reference points and reference planes can be used to produce the three-dimensional image from the autocorrelation of an image. However, it is not always possible to implement the reference-point techniques that currently exist in the art. There are situations where the imaging system may not be able to predetermine or place a reference point in proximity of the object. There are also situations where significant benefits can be gained from a movable and self contained imaging system that can create reference points.
Where reference points are not possible, and phase information of the received light is not known, speckle-pattern sampling requires demanding computational approaches for reconstructing the three-dimensional image from its autocorrelation. It has been an ongoing challenge to determine methods and systems to efficiently transform this autocorrelation data into a representation of an object.
Therefore, there exists a need in the art for three-dimensional imaging systems and methods that addresses these shortcomings.